The present invention relates to a method and apparatus for communication transmission between a master controller and a slave controller and more particularly to methods of intercommunication over a single communication line. The present invention also more particularly relates to methods and apparatus for preventing communication line blockage by a faulty slave controller, faulty information reception due to simultaneous slave controller transmission and due to simultaneous master controller reception and transmission.
Prior art has been faced with the problem of operating system devices at a point remote from the main control such as, environmental control systems, chemical process lines, etc. Previously, the inputs from process sensing elements, such as flow meters, pressure transducers or the like, and the outputs to the control elements, such as valves, motors or the like, were connected to closely coupled process peripheral devices comprising analog-to-digital and digital-to-analog converters. This resulted in long analog signal wiring and an increased load on the controller not only to perform the functions of the supervisory loop but also to monitor the secondary loop dynamic requirements such as tuning coefficients, scaling parameters, etc.
The contemporary solution provided by prior art to this problem is to provide a central controller, sometimes referred to as a master or host controller and slave controllers or central processing units locatd at a point in near proximity to the devices to be operated. Communications from the master controller to the multiple slave processors has been intensified during the past decade by the introduction of the microprocessor. The microprocesor allowed the secondary loop dynamic requirements to be removed from the master or host processor and distributed toward the control element. This distributed controller or slave controller contains the required analog-to-digital and digital-to-analog converter facility as well as the means of communicating with the master controller. The means of accomplishing this communication is typically referred to as a data bus or data highway over which information usually digitally encoded in a bit serial format is transmitted between the master controller and the slave controller. The master/slave dialog requires a half duplex mode of operation as the slaves speak only when spoken to by the master.
Prior art buses have been implemented by various base band types of integrated circuit driver/receiver elements to support such line specifications as EIA RS232C, or EIA RS422. Such drivers require a common ground reference from slave controller to slave controller the full length of the bus. Even the use of differentially coupled driver/receivers such as those supporting the RS422are limited to common mode ground noise levels of less than 10 to 20 volts. For this type of system, a single hose or master controller can only support 20 or 30 slave stations distributed over only distances of 1,000 feet.
The relatively low comon mode voltage and short bus length of prior art does not meet the needs of a distributed system in a current industrial environment. An additional problem of prior art is concerned with the faiure of one of the slave controllers. Typically, information exchanged between the master and slave controllers is in digital format over a single line for all slave controllers. Digital format consists of high and low signals, their sequential configuration determining the character transmitted. If the slave fails in the high mode, transmission between master controller and other slave controllers may be blocked when communication is over the same transmission line for a multiple of slave controllers. Thus, the communication between slaves and master is blocked in the high position preventing all communication between the master and the slaves.
Additional problems of the prior art master slave communication systems is concerned with the simultaneous communication from multiple slave controllers to the master controller, which garbles the information received by the master controller. Simultaneous communication errors may be similar to those caused by electrical noise.
It is therefore an object of the present invention to provide an inexpensive communication system between a master controller and multiple slave controllers in a multidrop configuration that is inherently immune from all forms of electrical noise.
It is also an object to provide a system which permits master and slave communications on a minimum number of control wires.
It is a further object to provide a system which prevents simultaneous mater and slave communications.
It is also a further object to provide a system which detects simultaneous communication of more than one slave.
It is another object to provide a system which will prevent a faulty slave controller from blocking information transmission.